The invention relates to a method for generating switching signals from AC current containing harmonics or from a mixture of at least two AC currents with harmonics and of adjacent fundamentals. Such currents may occur for instance when the positions of index marks are determined in one or several coordinates by means of periodic photoelectric scanning, each measured coordinate being associated with a fundamental frequency and the scanning signal being analysed by one or several discriminators.
There are three null values in the output signal of a discriminator, for instance in a photoelectric scanning circuit, as shown in West German Pat. Nos. 1,265,435 of F. Hock, published May 13, 1971 and 1,448,854 of K. Heinecke and W. Holle, published Aug. 23, 1970, the disclosures of which are incorporated herein, namely, one prior to and one after a measured index mark being in the range of scanning and one intermediate one, the latter representing coincidence between an index mark and a reference value. As regards the technique of measurement, it is only this latter null transit which is of interest, and the problem consists in distinguishing it from the others. This may be achieved in known manner by indexing only this particular null process by a switching signal restricted to the range of this particular null transition.
It is known this display signal is so connected logically with the output signal of the discriminator that there is no signal at the output of the latter from beyond the sensitivity range of the scanning device, that is, no interferences of any kind may be transmitted, such as transients, stray pulses, etc., which represents a considerable advantage for automatic measuring instruments or systems.
A method already known according to U.S. Pat. No. 3,454,776 where generation of the switching signals is based on the fact that the transition of an indexing mark through the scanning range results in a scanning signal frequency function f .fwdarw. 2f .fwdarw. f. The discriminator null transition of interest is in the 2f-range indicated by the frequency measurement.
The apparatus used in this prior art method suffers from its sensitivity to interferences, for instance noise superposed on the measured signal. In order to ensure that this apparatus will not respond to such interferences, hysteresis must be incorporated. The latter in turn reduces sensitivity and therefore such a system does not operate flawlessly below a certain amplitude of the scanned signal, for instance if the illumination is lowered during photoelectric scanning. The method furthermore suffers from the drawback of becoming inoperative if more than one fundamental is present in the scanned signal. Therefore, one may not achieve switching signals for two or more coordinates from correspondingly mixed frequencies when making use of such systems.